Physical Sciences Research Highlights

July 2016

The Case of the Cobalt Catalyst

The story's plotline could solve other mysteries around generating electricity without fossil fuels

It’s the worst short story ever written: on a dark and stormy night; the end. The real story – the context, the tension, and the motivations -- are missing. That’s what it feels like for scientists reading the reaction that uses a cobalt catalyst to produce hydrogen. Eric Wiedner and Morris Bullock at Pacific Northwest National Laboratory wanted to know the rest of the story. They found out what happened between the first page and the last. Image credit: J. Darmon, PNNLEnlarge image.

It's
the worst short story ever written: on a dark and stormy night; the end. The real
story -- the context, the tension, and the motivations -- are missing. That's
what it feels like for scientists reading the reaction that uses a cobalt
catalyst to produce hydrogen. Dr. Eric Wiedner and Dr. Morris Bullock at
Pacific Northwest National Laboratory (PNNL) wanted to know the rest of the
story. They found out what happened between the first page and the last.

The
challenge was that the intermediate steps, the middle of the story, occur
exceedingly fast. This left scientists studying possible routes, but not
knowing for sure what happened. To solve the mystery, Wiedner and Bullock used
newer techniques to show how the actors form bonds, trade electrons, and free
hydrogen.

"We
need to know how catalysts work in more detail, so we can change the catalyst to
work faster and more efficiently," said Wiedner, lead author on the study.

The scientists worked at the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center. The U.S. Department of
the Energy's Office of Science funds the centers to accelerate discovery. The
centers do so by combining scientific talent with powerful tools to understand and
manipulate matter on the atomic and molecular scales.

Why It Matters: Fuel cells offer a way to produce
electricity without using fossil fuels. Inside a fuel cell, a catalyst drives a
reaction that breaks apart hydrogen to produce electricity. Because a fuel cell
is rechargeable, the catalyst also "charges" the fuel cell by producing
hydrogen. The challenge is in the nuances of designing the catalyst. Wiedner
and Bullock focused on a cobalt catalyst and how it forms bonds with hydrogen
atoms and shuffles electrons. Changing how bonds form and electrons transfer
can cause a catalyst to struggle or succeed.

"You
don't focus on the areas where you can make incremental changes," said Bullock,
who directs the Center for Molecular Electrocatalysis. "You focus on the areas with the
big changes."

Methods: To improve the catalysts' ability and
speed up fuel cells, scientists need to know how to redesign catalysts and
adjust reactions. This often involves understanding the strength with which the
hydrogen binds to the catalyst. The tighter the bond, the more energy it takes
to free the hydrogen. It also involves the energy involved in shuffling
electrons. The less energy needed to move the electrons, the more efficient the
catalyst.

Bullock
and Wiedner began with a cobalt catalyst that drives hydrogen production. They
knew the reaction began with two electrons and two protons and produced
hydrogen (formula: H2). To uncover the intermediate steps, Wiedner
and Bullock combined two techniques: variable scan rate cyclic voltammetry and
foot-of-the-wave analysis, in PNNL's Physical Sciences Laboratory. These
electrochemical methods are an easily accessible, widely applicable way of
analyzing reactions. Using these methods, the scientists measured the sequence
of proton and electron transfers. They then calculated the various bond
strengths using density functional theory computations at the National Energy
Research Scientific Computing Center.

By
combining the experimental results and calculations, they filled in the missing
pieces of the story. Of all of the possible plotlines, the team found that this
is the story of the catalyst:

The
fuel cell's electrode delivers an electron to cobalt, making it more reactive.

Cobalt
grabs a proton from the surrounding liquid. This proton is chemically bound to the
catalyst's surface.

The
electrode feeds another electron to cobalt, causing it to share its extra
electrons with the bonded proton.

The
proton bound to cobalt steals two electrons from the metal and gives them to
another proton in solution, triggering the release of hydrogen and leaving
cobalt empty handed.

In
addition to determining the four-step process, where cobalt changes its
electronic structure several times, this research helps scientists with the
nuances of building better catalysts. By understanding the details of bond
formation, scientists can reduce the strength of the bonds and ease the
structural changes when electrons come in. These subtle changes can create a
different conclusion regarding the energy used and time taken.

What's Next? While this work wraps up a series of studies on
cobalt catalysts done at the Center for Molecular Electrocatalysis, it marks
the start of showing how scientists can use these electrochemistry techniques
on other catalysts.

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In short...

133-character summary: #EFRC scientists uncover the middle of
an important reaction; they show how the actors form bonds, trade
electrons, and free hydrogen

Short summary: Scientists at the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by DOE's Office of
Science, used recent electrochemical techniques and complex calculations to delve
into the reactions that produce hydrogen, a possible fuel source; they
demonstrated that a proposed key intermediate in hydrogen production is formed
and then reduced.